Liquid is supplied to the pulsing tool, typically from a reservoir at the surface. A pressurized volume of the liquid is contained in an accumulator, which may be regarded as including the volume contained in the pipe or conduit leading down, from the surface, to the pulse-tool.
The pulse-tool includes a pulse-valve, through which liquid passes from the accumulator into the formation when the pulse-valve is open. That flow is blocked when the pulse-valve is closed. Thus, the formation-pressure is rising when the pulse-valve is open, and the formation-pressure is falling when the pulse-valve is closed, when the just-injected liquid dissipates into the ground. Likewise, the accumulator-pressure is falling when the pulse-valve is open, and is rising (i.e the accumulator is recharging) when the pulse-valve is closed.
The frequency and magnitude of the pulses is affected by the back-pressure of the ground formation around the borehole. The formation-pressure rises/falls, and the accumulator-pressure falls/rises, when the pulse-valve is open/closed.
The pulse-valve operates automatically in response to changes in these pressures, and particularly in response to the changing differential pressure between the accumulator-pressure and the formation-pressure, herein termed the PDAF. When the pulse-valve is closed, the PDAF increases towards its high-threshold; when the pulse-valve is open, the PDAF decreases towards its low-threshold. The pulse-valve automatically cycles open-closed-open-closed-etc, so long as the conditions are such that the PDAF cycles between its high- and low-threshold levels.
The designers seek to open the pulse-valve very rapidly, because the resulting burst of energy can create a shock-wave that assists the pulse in travelling large distances through the ground. The more explosively the pulse-valve can open, the greater the energy of the resulting shock-wave, and the greater its penetration.